Why do I have a single vertex connected to 2 edge loops?
It's not a single vertex, non of your top and bottom of the barrel faces have UV coordinates.

If you move that "one single vertex" in the UV Image editor you will find that there are actually several overlapping vertex in the 0,0 coordinate, in fact if you move more of them you will start seeing faces appear which correspond to the bottom and top of your barrel.

In other words you have to unwrap the rest of the mesh also